Reducing traffic loss in an EAPS system

ABSTRACT

A ring network with an automatic protection switching domain includes a control VLAN and at least one data VLAN. A master node in the ring is connected to at least one transit node. Each node in the ring network is linked to an adjacent node by a primary port or a secondary port. The master node receives notification of a fault via the control VLAN, the fault indicating a failed link between adjacent nodes. In response, the master node unblocks its secondary port to traffic on the data VLAN(s). The forwarding database entries on the master node and on the transit node(s) are flushed. Data traffic is flooded to the ring network until forwarding database entries on the master node and on the transit node(s) have been reestablished.

FIELD

Embodiments of the invention relate to computer networks, and moreparticularly to automatic protection switching in a ring network.

BACKGROUND

In an Ethernet automatic protection switching (EAPS) system, loops areprevented in a layer-2 network having a ring topology. An EAPS domainincludes a control virtual local area network (VLAN) and at least oneprotected data VLAN. The EAPS domain is associated with a master nodewhich is linked to at least one transit node in a ring network.

When a network failure is detected on the ring, the master node in anEAPS system receives control messages over the control VLAN, the controlmessages indicating the network failure. During normal operation, themaster node blocks the protected data VLAN traffic from traversing itssecondary port. During a network failure, the master node unblocks itssecondary port and reroutes the protected data VLAN traffic through itssecondary port. The secondary port is re-blocked once the failure hasbeen fixed. Each time the secondary port is blocked and unblocked, theforwarding databases on all the nodes in the ring are flushed.

Flushing the forwarding databases on the nodes requires that theforwarding entries and/or paths in each of the forwarding databases berelearned (e.g., automatically relearned) and/or reprogrammed (e.g.,manually reprogrammed), both of which are expensive. In either case,reestablishing the forwarding databases does not occur instantaneously.In other words, there is period of time between the commencement offlushing the forwarding databases and subsequently reestablishing them.During this period of time, network connectivity can be temporarilysevered and data (e.g., data frames, packets, etc.) can be dropped orlost.

SUMMARY OF THE INVENTION

A ring network with an automatic protection switching domain includes acontrol VLAN and at least one data VLAN. A master node in the ring isconnected to at least one transit node. Each node in the ring network islinked to an adjacent node by a primary port or a secondary port. Duringnormal operation, the master node allows data traffic to flow throughits primary port while its secondary port is blocked. By blocking thesecondary port, the master node prevents a loop. When a fault isdetected on the ring, the fault is communicated to the master node viathe control VLAN. A fault indicates a failed link between adjacentnodes. In response to the fault, the master node unblocks its secondaryport to traffic on the data VLAN(s).

The change in the flow of traffic causes existing forwarding databaseentries on the master node and on the transit node(s) to be invalid.Thus, the forwarding database entries are flushed from the master nodeand the transit node(s). Flushing the forwarding database entriesnecessitates relearning of forwarding routes based on the newconfiguration of the ring network (i.e., master node secondary portunblocked). During the period of time between flushing the entries andrelearning the entries, data traffic is flooded to the ring network tomaintain connectivity and prevent frame and/or packet loss. Once theforwarding database entries are relearned, the traffic floodingautomatically stops.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description includes discussion of figures havingillustrations given by way of example of implementations of embodimentsof the invention. The drawings should be understood by way of example,and not by way of limitation. As used herein, references to one or more“embodiments” are to be understood as describing a particular feature,structure, or characteristic included in at least one implementation ofthe invention. Thus, phrases such as “in one embodiment” or “in analternate embodiment” appearing herein describe various embodiments andimplementations of the invention, and do not necessarily all refer tothe same embodiment. However, they are also not necessarily mutuallyexclusive.

FIG. 1 illustrates a prior art Ethernet automatic protection switching(EAPS) system.

FIG. 2A illustrates a state of an EAPS ring network during normaloperation according to various embodiments.

FIG. 2B illustrates a state of an EAPS ring network immediately after alink failure according to various embodiments.

FIG. 2C illustrates a state of an EAPS ring network after failoveraccording to various embodiments.

FIG. 3 is a flow diagram illustrating a process for forwarding datatraffic according to various embodiments.

FIG. 4 is a block diagram illustrating a suitable computing environmentfor practicing various embodiments.

DETAILED DESCRIPTION

As provided herein, methods, apparatuses, and systems prevent maintainconnectivity and prevent frame and/or packet loss upon the occurrence ofa fault on a ring network that employs Ethernet automatic protectionswitching (EAPS).

FIG. 1 is a block diagram illustrating a prior art EAPS system. The EAPSsystem 100 consists of one or more EAPS domains 101. A control VLAN 103is created for each EAPS domain 101 for the purpose of sending andreceiving EAPS system control messages 117. The EAPS domain 101 iscreated to protect a group of one or more data carrying VLANs 104.

The EAPS system 100 operates on a ring network 102. One node on the ringnetwork 102 is designated as the master node 105. The two ring ports onthe master node 105 are designated as primary port 106 and secondaryport 107. All other nodes on the ring network 102 are transit nodes 111and each has its respective ring ports 112. Each master node 105 andtransit node 111 has a forwarding database (FDB), 108 and 113respectively, in which they store information about the networkcommunication paths. The master node 105 has a state register 109 forstoring the state of the ring network 102. For the purpose ofillustration, the states of the ring network 102 are described either as“failed,” meaning there is a fault or break in the ring network 102, oras “complete,” meaning that the ring network is unbroken or the ringnetwork has been restored and all nodes are communicating correctly. Thetransit nodes 111 have a state register 114 in which they store thepre-forwarding state, and a pre-forwarding timer 115. The transit nodes111 also have a temporarily-blocked-port storage area (TBP) 116 in whichthey store the identification of the port that is temporarily blocked.Pre-forwarding and TBPs are discussed below.

In various embodiments, the master node 105 and the transit nodes 111use control messages 117 to communicate via the control VLAN 103. Someexamples of control messages 117 in embodiments are health-checkmessages, link-down messages, and flush-FDB messages. The transit node111 recognizes a message sent on the control VLAN 103 as a controlmessage 117 because it has a special MAC (media access control) addressthat corresponds to an entry in the forwarding database 113. The masternode and the transit nodes forward the control message 117 prior tocopying it to the central processing unit (CPU) of the node where, amongother things, it is logged for use in troubleshooting. Forwarding thecontrol message 117 before processing by the CPU facilitates theconvergence of the ring network 102 after a fault in substantially lesstime than can be achieved with previous prior art methods.

The master node 105 has a hello-timer 118, which is the clock forsending the health-check control messages 117. Once the hello-timer 118is started, it prompts the master node 105 to send a health-checkmessage 117 on the control VLAN 103 at regular intervals, for exampleevery one second. The health-check message 117 is forwarded around thering network 102 and returns to the master node 105 nearlyinstantaneously. When the master node 105 sends the health-check message117, it sets the fail-timer 110. Should the fail-timer 110 expire beforethe health-check message is returned to the master node 105, the masternode 105 determines that there is a fault in the ring network 102. Thehealth-check messages 117 are sent even during a fault. When the faultis restored, the master node 105 knows immediately because the return ofthe health-check message 117 is resumed.

As used herein, the time it takes to bring a network from a failed stateto a complete state, is referred to as the “failover” time. In otherwords, the failover time is the time it takes to relearn the FDB entriesafter they have been flushed. While the flushing of FDB entries isappropriately associated with link failure or other fault, flushing canalso occur intentionally (e.g., for network maintenance, etc.). Giventhat a system might include 100,000 hosts or more, each having aseparate route entry, the failover time can be significant from anetwork connectivity perspective. Prior art FDB entries simply define anoutput port through which data should be forwarded to reach a particulardestination. During a failover period, many of these FDB entries canbecome invalid because the path through the defined output port mayinclude the failed link. Invalid FDB entries can cause data traffic(e.g., frames, packets, etc.) to be lost or dropped. In order to reducethe frame and/or packet loss in an EAPS system during failover, anadditional field, referred to herein as a fast-switchover (FSWO) field,is added to FDB entries in the FDB database(s).

In various embodiments, the FSWO field is a 1-bit field added to an FDBentry, though the field could be multiple bits in different embodiments.The FSWO field indicates whether the output port defined in the FDBentry is susceptible to invalidity during failover. Thus, the FSWO bitis set high (e.g., set to 1) for FDB entries that are susceptible toinvalidity. A low (e.g., 0) FSWO bit could be defined as indicating thatan FDB entry is susceptible to invalidity in other embodiments. Invarious embodiments, the FSWO field is examined whenever an FDB entry isaccessed. It is not important whether the FSWO field is examined beforeor after the output port field of an FDB entry.

When the FSWO bit of an entry is set high, indicating that the entry issusceptible to being invalid, a special port array is accessed. The portarray is maintained in hardware for fast access (e.g., in an internalRAM, etc.). More specifically, the port array is a global resource andallows for memory indexing, as opposed to requiring a search for aparticular port. Memory indexing with the port array minimizes hardwareresources because each EAPS port requires only a single bit in the portarray as opposed to 9, 13, or 16 bits, etc., for searching each entry,depending on the bit length of an FDB entry. The port array could bemaintained in software in other embodiments.

In various embodiments, the port array includes one bit in the array foreach port in the EAPS system. For example, a system having 512 portswould have a port array with 512 bits (e.g., [0:511]). Whenever the EAPSsystem detects a fault or link failure, the port array is modified bysetting the bit for each of the EAPS-controlled ports to high (e.g., 1).A high bit in the port array indicates the need to flood traffic to allrelevant EAPS ports. For example, a high bit in the port array for portX signifies that all traffic that would normally be routed through portX should be flooded to all ports. In this way, frames and/or packetsthat would otherwise be dropped or lost are able to reach theirdestination. Flooding on relevant EAPS ports occurs as long as the oneor more bits in the port array are high. When a failover period hasended (i.e., the FDB entries have been relearned and/or reprogrammed toaccount for the link failure), the port array bits are set low (e.g., to0). In other embodiments, the port array bits can be set low upon theexpiration of a timer. Thus, subsequent accesses to the port array(based on a high FSWO bit in the FDB entry) will indicate that floodingis not necessary.

FIGS. 2A-2C illustrate a ring network incorporating an EAPS system. Inparticular, FIG. 2A illustrates ring network 200 during normaloperation. Master node 210 has a primary port (P1) and a secondary port(P2). According to the EAPS system configuration, traffic on the dataVLAN is blocked from flowing through port P2. Thus, all traffic frommaster node is forwarded through port P1. The process for making aforwarding decision is illustrated by way of example below.

In this example, an incoming frame 202 arrives at master node 210 havinga destination of transit node 230 (address: XYZ). The FDB entry shown inFIG. 2A illustrates the forwarding decision process. When incoming frame202 arrives at master node 210, the entry corresponding to thedestination address XYZ is accessed. Upon accessing the entry, theoutput port field is examined and determined to be port P1. However, theprocess does not end after determining the output port. Subsequent todetermining the output port, the fast-switchover (FSWO) bit in the FDBentry is examined. If the FSWO bit is set high (e.g., 1), then the P1bit of the EAPS system port array 250 must be checked. Given that the P1bit in port array 250 is low (e.g., 0) in FIG. 2A, flooding is notwarranted. Thus, master node 210 proceeds to forward incoming frame 202on port P1 as indicated in the FDB entry.

Continuing with the example, FIG. 2B illustrates the state of the ringnetwork 200 immediately after a link failure between transit nodes 220and 230. The link failure is communicated to master node 210 via theEAPS system control VLAN. Additionally, the port array 250 for the EAPSsystem is updated so that the bits corresponding to EAPS ports are sethigh (e.g., 1). In response to the detected link failure, master node210 unblocks its secondary port P2. Thus, when incoming frame 204,destined for transit node 230, arrives at master node 210, the FDBentry, as shown in FIG. 2B, erroneously indicates that the frame shouldbe forwarded on port P1. This is ok, however, because the query does notend after examining the output port field. After the output port fieldis examined, the P1 bit of port array 250 is examined because the FSWObit of the FDB entry is set high (e.g., 1). Here, the P1 bit of portarray 250 is also set high, having been updated on account of the linkfailure. A high bit in port array 250 indicates that the incoming frameshould be flooded on all relevant EAPS ports. Thus, the frame 204 isflooded out on both ports of master node 210 (i.e., P1 and P2). If notfor the flooding, the frame 204 would have been routed out on port P1,as directed by the FDB entry, and would have not reached transit node230 because of the failed link between transit nodes 220 and 230.However, because frame 204 was flooded on both ports P1 and P2, theframe now has a path through port P2 to its destination (i.e., transitnode 230).

FIG. 2C illustrates ring network 200, the FDB entry 212, and the portarray 250 after the EAPS-related FDB entries have been flushed andrelearned. When incoming frame 206, destined for transit node 230,arrives at master node 210, FDB entry 212 now correctly indicates thatthe frame should be forwarded on port P2. The FSWO bit is still sethigh, indicating the need to check port array 250 before forwarding theframe on port P2. Given that the EAPS-related FDB entries have beenrelearned to reflect the current state of the ring network 200, the bitsin port array 250 are set low (e.g., to 0). Thus, flooding is notnecessary and master node 210 can proceed to forward frame 206 on portP2 towards transit node 230.

FIG. 3 is a flow diagram illustrating a process for forwarding a dataframe and/or packet on an EAPS-enabled ring network according to variousembodiments. When a frame is received at a node other than its finaldestination, an FDB entry is retrieved 310 based on the frame'sdestination address (e.g., MAC address, IP address, etc.). An outputport field in the FDB entry indicates the port on which the frame shouldbe forwarded in order to reach its destination. Thus, the output port isdetermined 320. The FDB entry also includes a fast-switchover (FSWO)field (e.g., a 1-bit field). If it is determined 330 that the FSWO bitis set high (e.g., equal to 1), then it is necessary to check the portarray 340. This is because entries with a high FSWO bit may becomeinvalid in the event of a link failure or other fault on the ringnetwork. The port array is updated to indicate the existence of a linkfailure or fault and the need to compensate for it. Thus, the port arrayis checked 340 to determine 360 if the port array bit corresponding tothe output port (determined in step 320) is set high (e.g., equal to 1).If the bit is not set high, then the frame is forwarded 350 through theoutput port (determined in step 320). However, if the bit is set high,then a fault or failure condition has occurred; thus, the frame isflooded 370 on the relevant EAPS ports.

Those skilled in the art will appreciate that various alternateembodiments can be practiced in addition to the embodiments describedabove. For example, while the embodiments described above are generallyapplicable to layer 2 (e.g., data link layer), it is also contemplatedthat embodiments could be implemented using layer 3 (e.g., networklayer). For example, rather than having a MAC address as the destinationin an FDB entry (layer 2), an Internet Protocol (IP) address could beused as the destination address in a layer 3 FDB entry. In layer 3embodiments, FDB entries might include a VLAN field in addition to theoutput port field because the output VLAN might be different than theinput VLAN in layer 3. Additionally, embodiments are not limited tounicast traffic or single path routing; multi-cast traffic and equalcost multi-path routing (ECMP) can be used in other embodiments.

FIG. 4 illustrates computing environment in which certain aspects of theinvention illustrated in FIGS. 1-3 may be practiced in variousembodiments. Collectively, these components are intended to represent abroad category of hardware systems, including but not limited to generalpurpose computer systems and specialized network switches.

Computer system 400 includes processor 410, I/O devices 440, main memory420 and flash memory 430 coupled to each other via a bus 480. Mainmemory 420, which can include one or more of system memory (RAM), andnonvolatile storage devices (e.g., magnetic or optical disks), storesinstructions and data for use by processor 410. Additionally, thenetwork interfaces 470, data storage 460, and switch fabric 450 arecoupled to each other via a bus 480. Data storage 460 represents therouting database (i.e., route tables) described herein as well as otherstorage areas such as packet buffers, etc., used by the switch fabric450 for forwarding network packets or messages.

The various components of computer system 400 may be rearranged invarious embodiments, and some embodiments may not require nor includeall of the above components. Furthermore, additional components may beincluded in system 400, such as additional processors (e.g., a digitalsignal processor), storage devices, memories, network/communicationinterfaces, etc.

In the illustrated embodiment of FIG. 4, the method and apparatusreducing route table size according to the present invention asdiscussed above may be implemented as a series of software routines runby computer system 400 of FIG. 4. These software routines comprise aplurality or series of instructions to be executed by a processingsystem in a hardware system, such as processor 410. Initially, theseries of instructions are stored on a data storage device 460 (e.g., ina route manager database), memory 420 or flash 430.

Various components described herein, including the components of FIG. 4,may be a means for performing the functions described herein. In a casewhere a component to perform operations described herein includessoftware, the software data, instructions, and/or configuration may beprovided via an article of manufacture by a machine/electronicdevice/hardware. An article of manufacture may include a computerreadable medium having content to provide instructions, data, etc. Thecontent may result in an electronic device as described herein,performing various operations or executions described. A computerreadable medium includes any mechanism that provides (i.e., storesand/or transmits) information/content in a form accessible by a computer(e.g., computer, computing device, electronic device, electronicsystem/subsystem, etc.). For example, a computer readable mediumincludes recordable/non-recordable media (e.g., read only memory (ROM),random access memory (RAM), magnetic disk storage media, optical storagemedia, flash memory devices, etc.). The computer readable medium mayfurther include an electronic device having code loaded on a storagethat may be executed when the electronic device is in operation. Thus,delivering an electronic device with such code may be understood asproviding the article of manufacture with such content described herein.Furthermore, storing code on a database or other memory location andoffering the code for download over a communication medium may beunderstood as providing the article of manufacture with such contentdescribed herein.

Besides what is described herein, various modifications may be made tothe disclosed embodiments and implementations of the invention withoutdeparting from their scope. Therefore, the illustrations and examplesherein should be construed in an illustrative, and not a restrictivesense. The scope of the invention should be measured solely by referenceto the claims that follow.

1. A method comprising: retrieving a forwarding database (FDB) entryfrom a forwarding device on a ring network with an automatic protectionswitching domain; determining an output port associated with the FDBentry; inspecting a field in the FDB entry, the field to indicatewhether the output port is susceptible to invalidity upon a fault in thering network; inspecting a port field associated with the output port ina port array, wherein inspecting the port field is in response toinspecting the field in the FDB entry; detecting a fault in the ringnetwork, the ring network having a master node connected to at least onetransit node, each node linked to an adjacent node by at least one of aprimary port or a secondary port, the fault indicating a failed linkbetween adjacent nodes; unblocking the secondary port of the master nodein response to detecting the fault; and flooding traffic through allcommunicating ports in the ring network in response to inspecting theport field in the port array and unblocking the secondary port, thecommunicating ports including the primary port and the unblockedsecondary port of the master node, wherein flooding the traffic untilrespective forwarding database entries on the master node and on the atleast one transit node have been reestablished.
 2. The method of claim1, further comprising flushing forwarding database entries on the masternode and on the at least one transit node.
 3. The method of claim 1,wherein the field in the FDB entry to indicate whether the output portis susceptible to invalidity upon a fault in the ring network is afast-switchover.
 4. The method of claim 1, wherein the forwarding deviceis one of a layer 2 switch and a layer 3 router.
 5. The method of claim1, wherein inspecting the port field associated with the output port inthe port array comprises inspecting the port field when the field in theFDB entry indicates that the output port is susceptible to invalidity.6. An article of manufacture comprising a computer-readable storagemedium having content stored thereon to provide instructions that whenexecuted result in an electronic device performing operations including:retrieving a forwarding database (FDB) entry from a forwarding device ona ring network with an automatic protection switching domain;determining an output port associated with the FDB entry; inspecting afield in the FDB entry, the field to indicate whether the output port issusceptible to invalidity upon a fault in the ring network; inspecting aport field associated with the output port in a port array, whereininspecting the port field is in response to inspecting the field in theFDB entry; detecting a fault in the ring network, the ring networkhaving a master node connected to at least one transit node, each nodelinked to an adjacent node by at least one of a primary port or asecondary port, the fault indicating a failed link between adjacentnodes; unblocking the secondary port of the master node in response todetecting the fault; and flooding traffic through all communicatingports in the ring network in response to inspecting the port field inthe port array and unblocking the secondary port, the communicatingports including the primary port and the unblocked secondary port of themaster node, wherein flooding the traffic until respective forwardingdatabase entries on the master node and on the at least one transit nodehave been reestablished.
 7. The article of manufacture of claim 6,wherein the computer readable storage medium having content storedthereon to provide instructions that when executed result in theelectronic device performing further operations including flushingforwarding database entries on the master node and on the at least onetransit node.
 8. A system comprising: a memory to store a forwardingtable associated with a forwarding device; means for retrieving aforwarding database (FDB) entry from the forwarding device on a ringnetwork with an automatic protection switching domain; means fordetermining an output port associated with the FDB entry; means forinspecting a field in the FDB entry, the field to indicate whether theoutput port is susceptible to invalidity upon a limit in the ringnetwork; means for inspecting a port field associated with the outputport in a port array, wherein inspecting the port field is in responseto inspecting the field in the FDB entry; means for detecting a fault ina ring network, the ring network having a master node connected to atleast one transit node, each node linked to an adjacent node by at leastone of a primary port or a secondary port, the fault indicating a failedlink between adjacent nodes; means for unblocking the secondary port ofthe master node in response to detecting the fault; and means forflooding traffic through all communicating ports in the ring network inresponse to inspecting the port field in the port array and unblockingthe secondary port, the communicating ports including the primary portand the unblocked secondary port of the master node, wherein the meansfor flooding to flood the traffic until respective forwarding databaseentries on the master node and on the at least one transit node havebeen reestablished.
 9. The system of claim 8, wherein the forwardingdevice is one of a layer 2 switch and a layer 3 router.
 10. The systemof claim 8, wherein the field in the FDB entry to indicate whether theoutput port is susceptible to invalidity upon a fault in the ringnetwork is a fast-switchover field.
 11. The system of claim 8, whereinthe means for inspecting the port field associated with the output portin the port array further comprises means for inspecting the port fieldwhen the field in the FDB entry indicates that the output port issusceptible to invalidity.
 12. A method comprising: retrieving aforwarding database (FDB) entry from a forwarding device on a ringnetwork with an automatic protection switching domain; determining anoutput port associated with the FDB entry; inspecting a field in the FDBentry, the field to indicate whether the output port is susceptible toinvalidity upon a fault in the ring network; inspecting a port fieldassociated with the output port in a port array, wherein inspecting theport field is in response to inspecting the field in the FDB entry; andflooding traffic to ring ports associated with the automatic protectionswitching domain responsive to the inspected port field associated withthe output port in the port array.
 13. The method of claim 12, whereinthe field in the FDB entry to indicate whether the output port issusceptible to invalidity upon a fault in the ring network is afast-switchover field.
 14. The method of claim 12, wherein inspectingthe port field associated with the output port in the port arraycomprises inspecting the port field when the field in the FDB entryindicates that the output port is susceptible to invalidity.
 15. Themethod of claim 12, wherein the field to indicate whether the outputport is susceptible to invalidity upon a fault in the ring network is a1-bit field.
 16. The method of claim 12, wherein the port array ismaintained in a random access memory (RAM).
 17. The method of claim 12further comprising modifying the port array when a fault in the ringnetwork is detected.
 18. The method of claim 12, wherein the forwardingdevice is one of a layer 2 switch and a layer 3 router.
 19. An articleof manufacture comprising computer readable storage medium havingcontent stored thereon to provide instructions that when executed resultin an electronic device performing a method comprising: retrieving aforwarding database (FDB) entry from a forwarding device on a ringnetwork with an automatic protection switching domain; determining anoutput port associated with the FDB entry; inspecting a field in the FDBentry, the field to indicate whether the output port is susceptible toinvalidity upon a fault in the ring network; inspecting a port fieldassociated with the output port in a port array, wherein inspecting theport field is in response to inspecting the field in the FDB entry; andflooding traffic to ring ports associated with the automatic protectionswitching domain responsive to the inspected port field associated withthe output port in the port array.
 20. The article of manufacture ofclaim 19, wherein the field in the FDB entry to indicate whether theoutput port is susceptible to invalidity upon a fault in the ringnetwork is a fast-switchover.
 21. The article of manufacture of claim19, wherein inspecting the port field associated with the output port inthe port array comprises inspecting the port field when the field in theFDB entry indicates that the output port is susceptible to invalidity.22. The article of manufacture of claim 19, wherein the field toindicate whether the output port is susceptible to invalidity upon afault in the ring network is a 1-bit field.
 23. The article ofmanufacture of claim 19, wherein the port array is maintained in arandom access memory (RAM).
 24. The article of manufacture of claim 19with computer readable storage medium having content stored thereon toprovide further instructions that when executed result in the electronicdevice performing a further method comprising, modifying the port arraywhen a fault in the ring network is detected.
 25. The article ofmanufacture of claim 19, wherein the forwarding device is one of a layer2 switch and a layer 3 router.
 26. An apparatus comprising: logic forretrieving a forwarding database (FDB) entry from a forwarding device ona ring network with an automatic protection switching domain; logic fordetermining an output port associated with the FDB entry; logic forinspecting a field in the FDB entry, the field to indicate whether theoutput port is susceptible to invalidity upon a fault in the ringnetwork; logic for inspecting a port field associated with the outputport in a port array, wherein inspecting the port field is in responseto inspecting the field in the FDB entry; and logic for flooding trafficto ring ports associated with the automatic protection switching domainresponsive to the inspected port field associated with the output portin the port array.
 27. The apparatus of claim 26, wherein the field inthe FDB entry to indicate whether the output port is susceptible toinvalidity upon a fault in the ring network is a fast-switchover field.28. The apparatus of claim 26, wherein the logic for inspecting the portfield associated with the output port in the port array comprises logicfor inspecting the port field when the field in the FDB entry indicatesthat the output port is susceptible to invalidity.
 29. The apparatus ofclaim 26, wherein the field to indicate whether the output port issusceptible to invalidity upon a fault in the ring network is a 1-bitfield.
 30. The apparatus of claim 26, wherein the port array ismaintained in a random access memory (RAM).
 31. The apparatus of claim26 further comprising logic for modifying the port array when a fault inthe ring network is detected.
 32. The apparatus of claim 26, theforwarding device is one of a layer 2 switch and a layer 3 router.